This invention relates to polymeric lubricating oil additives containing phenolic antioxidant groups, more particularly polymeric additives which combine effective antioxidant properties with viscosity index (VI) improvement in high temperature service.
It is well-known that lubricants tend to oxidize when exposed to adverse conditions. In the internal-combustion engine, the temperature of operation is an important variable since the oxidation rate increases with an increase in temperature. The formulation of lubricants to meet the most severe type of engine operation has been a great challenge, particularly in the area of multigraded oils for diesel engine applications. Additionally, with higher temperature experienced in gasoline engines resulting from use of pollution control equipment and a trend to small, high-revving engines, greater importance is now attached to more thermally and oxidatively stable lubricants for these applications. Thus, fluids which are conventionally conceived for mild diesel applications may find utility in the most severe gasoline engine operations.
Diesel engines, which are used to power a variety of heavy duty vehicles, such as trucks, buses, farm tractors, construction equipment and trains, generally impose severe high temperature service on lubricants. The temperatures in the upper ring belt zone of diesel pistons reach 500.degree.-700.degree. F in mild and high super-charged versions whereas temperatures in gasoline engines generally do not exceed 500.degree. F.
Correct operation of the diesel engine requires that the piston be properly lubricated and remain free to travel unimpeded in the cylinder, compression being maintained by means of the secure-fitting rings located in the piston grooves. These rings act as lubrication pumps and must be able to move vertically as well as circumferentially in the groove. When the fluid is exposed to the high temperatures of the top groove, decomposition occurs proportionate to the thermal/oxidative stability of the components of the fluid. This decomposition results in resinous, carbonaceous deposits in the groove(s) and on the lands of the piston. If sufficient buildup of deposits occurs, ring sticking results with eventual compression loss, piston scuffing or piston seizure. Thus, an important element in the quality control for a diesel fluid is the specification of a maximum amount of deposit in the top groove and other locations.
Historically, single-grade oils have occupied a preeminent position as recommended fluids for diesel engine application, particularly 30 and 40 weight oils whose base stocks are less volatile than lower SAE grades and would be less likely to volatilize in the hot zones of the piston. Multigraded diesel oils have not achieved acceptance, although diesel engine operation in cold weather would indicate a significant advantage with multigrades. Diesel engines are usually difficult to start, especially in cold weather. The cold-start problems are now remedied (1) by the use of external heaters, an added expense and inconvenience; (2) by the use of ether, resulting in engine damage since this extremely volatile solvent provides an explosive start with piston movement in the absence of lubrication; or (3) by permitting continuous running when not in use, a situation which is wasteful of fuel and harmful from an environmental standpoint. Environmental and energy related considerations, therefore, could lead to a significant demand for multigraded diesel lubricants.
It is widely believed in the petroleum industry that the presence of polymer degrades diesel performance by increasing the amounts of Top Groove Fill (TGF) and lacquer. Their deficiency is the principal reason today that less than ten percent of the diesel lubricants used in this country are multigraded. For the major market, two performance levels are defined, Classification CC (Caterpillar 1-H) and Classification CD (Caterpillar 1-G), with the 1-G level representing more severe, highly super-charged, engine operation. For level CC, multigrade oils are available but they are not widely used due to higher costs and general reluctance to the use of polymer. Today, there are no multigraded level CD lubricants, and the technology for formulating them in a completely satisfactory way is not known. A new polymer providing completely acceptable multigraded 1-H and 1-G performance without introducing a severe cost penalty would represent a major breakthrough.